Modeling Coastally Trapped Wind Surges Over Southeastern Australia. Part II: Intensity and Depth

174 WEATHER AND FORECASTING VOLUME 15 Modeling Coastally Trapped Wind Surges over Southeastern Australia. Part II: Intensity and Depth HELEN J. REID School of Mathematics, University of New South Wales, Sydney, Australia (Manuscript received 17 May 1999, in ®nal form 3 November 1999) ABSTRACT A numerical weather prediction (NWP) model at the School of Mathematics, University of New South Wales, has been used to simulate the southerly buster, a southerly wind surge along the coast of New South Wales (NSW), which occurs during the spring and summer months. Three southerly buster events were simulated and comparison of the model results with observational data demonstrated that the NWP model was very good in simulating this type of event. These simulations were then used to consider the intensity, depth, and location of the southerly buster surges as they progressed northward. Both the strength and depth of the southerly buster surge decreased as it progressed farther north, particularly at the Hunter Valley. However, north of the Hunter Valley the intensity and depth increased again (in two of the three cases in this study) before dissipating completely. The location of the central part of the jet is adjacent to the Great Dividing Range, and the central jet splits at the northern side of the Hunter Valley to ¯ow around either side of the mountains. The southerly ¯ow into the Hunter Valley generally occurs before the continuation of the southerly buster up the coast. Sensitivity experiments, in which the topography at and north of the Hunter Valley was altered, were designed so that the southerly buster surge was able to develop naturally and then encounter different topographical features. These experiments indicate that the southerly buster is weaker and slower due to the natural break in the mountain barrier at the Hunter Valley and that the mountains to the north of the Hunter Valley act to delay the movement farther north with indications of reintensi®cation in the region. It is concluded that the topography has a signi®cant effect on the propagation of the southerly buster. 1. Introduction signi®cant consequences, but there is little documen- tation about this. The questions of how far north the The southerly buster is a strong, sudden, squally, southerly buster goes, and whether or not it reach Coffs southerly wind surge along the New South Wales Harbour, north of the Hunter Valley, are of great interest (NSW) coast in spring and summer. There have been for various reasons, for example, relief from hot weather many studies (see, e.g., McInnes and McBride 1993) on and planning alternatives for weather-dependent activ- the development of the southerly buster but few on the ities. Likewise, the idea of a split in the surge with the propagation farther north. A closer look at the intensity cool, southerly wind ¯owing up the Hunter Valley may and depth of the southerly buster would also provide answer in part the question of how far inland does the useful information to assist the dif®cult problem of fore- southerly buster penetrate. The split ¯ow is of interest casting these surges. Intensity is a very important aspect to those in the Hunter Valley, much as it is for those of the surge as it is the wind strength that is felt and on the coast north of the Hunter Valley. causes the damage. An indication of intensity prior to An overview of southerly busters is presented in sec- a southerly buster event would enable precautionary tion 2 and three case studies have been included in measures to be taken to minimize damage. The depth section 3. The 27 February 1998 event is covered in of the surge is also of interest, particularly to aircraft more detail and is of a more ``classical'' nature than the potentially ascending or descending through the tur- events of 18 January 1998, when the southerly buster bulent surge. It is well known that the southerly buster did not penetrate as far north as Coffs Harbour, and of is trapped against the Great Dividing Range and the 28 November 1997 with the southerly buster occurring break in the barrier at the Hunter Valley would have earlier in the day resulting in a warmer atmosphere for the surge to ¯ow into on the north coast. The high-resolution (HIRES) numerical weather pre- Corresponding author address: Helen J. Reid, School of Mathe- diction (NWP) model used to simulate the case studies matics, UNSW, Sydney 2052, Australia. is detailed in section 4. Section 5 details model veri®- E-mail: [email protected] cation for the three case studies with the simulated re- q 2000 American Meteorological Society Unauthenticated | Downloaded 10/01/21 02:09 AM UTC APRIL 2000 REID 175 sults for intensity, depth, location, and split at the Hunter Valley in section 6. Sensitivity experiments involving the topography to consider the effect of the orientation and location of the mountain barrier on the impinging surge are contained in section 7. Section 8 presents our conclusions. 2. The southerly buster of southeast Australia Southerly busters occur during the spring and summer months in the southeast of Australia along the eastern side of the Great Dividing Range. They are a strong, sudden and squally southerly wind surge of at least 15 ms21 and have been known to gust up to 35 m s21 (Colquhoun et al. 1985), and are con®ned to the coastal regions, trapped against the mountains by the Coriolis force. The depth of the surge is generally less than 1 km and therefore is below the average height of the Great Dividing Range. The passage of the southerly buster is occasionally accompanied by a roll cloud and is notable, not only for the common wind shift from northwesterly to southerly, but also for the sudden drop of temperature that can be up to 158C within minutes (Colquhoun et al. 1985). A signi®cant rise in the sea level pressure (SLP) occurs as a ridge of high pressure follows the cold frontal system but precipitation is not FIG. 1. A map of southeast Australia over the domain of the 30- usual. km resolution model simulation indicating the location of the Great A southerly buster is generated when a cold front is Dividing Range (shaded region over 500 m), New South Wales, Vic- blocked and experiences anticyclonic deformation near toria, Queensland, the Hunter Valley, Tasmania, and the Tasman Sea. the southern parts of the Great Dividing Range in Vic- toria (McInnes 1993). The surge of air propagates north- Scone, Taree, and Coffs Harbour. All these locations are ward as a coastally trapped orographic jet up the east on the coast of NSW, except Cessnock and Scone, which coast of Australia, the duration usually being about 24 are in the Hunter Valley. Figure 2 illustrates the loca- h (Baines 1980), from the time the cold front reaches tions of these places; they are referred to in order of the Great Dividing Range to its dissipation on the north northward progression from 1 through 8. coast of NSW, or on the southern coast of Queensland For the three case studies presented, a broadscale syn- (McInnes and McBride 1993). See Fig. 1 for the lo- optic chart is included. While these are not able to show cations of these regions. the details of the southerly busters, they do provide In Reid and Leslie (1999), the focus was on the timing overviews of the synoptic situations. The time of these and speed of propagation of the southerly buster along charts is 0900. [All times referred to in this study are the NSW coast. Twenty southerly busters were chosen in Australian eastern standard time (UTC 1 1000), un- and HIRES demonstrated the ability to simulate the gen- less otherwise stated.] Comments on the synoptic sit- eral southerly buster ¯ow with regard to the SLP pattern uation are based on 3-hourly hand analyses from the and the characteristic S shape forming in the cold front. Australian Bureau of Meteorology, Sydney of®ce. The time of arrival of the southerly buster and the as- sociated speed of propagation were covered in more detail. Speci®c locations were chosen for the veri®cation a. The 27 February 1998 event of the simulated time of arrival of the southerly buster The event of the 27 February occurs near the end of and it was shown that the model was very good in this the southerly buster season. This particular event moved respect. The model also demonstrated skill in simulating up the southern coast of NSW during the morning to the speed of propagation. reach the region just south of Sydney at about 1300 and continued up the central coast during the afternoon, but did not reach the northern parts of the NSW coast until 3. Case studies the morning of 28 February 1998. Observational data of various forms from the Bureau Analysis charts indicate that a cold frontal system in of Meteorology have been collected for eight veri®ca- the eastern bight region at 1700 26 February 1998 con- tion locations. From south to north, these are Bellambi, tinued east until it had crossed over northeast NSW by Fort Denison, Norah Head, Williamtown, Cessnock, 1400 28 February 1998, a period of approximately 2 Unauthenticated | Downloaded 10/01/21 02:09 AM UTC 176 WEATHER AND FORECASTING VOLUME 15 FIG. 3. SLP chart for 0900 27 Feb 1998. 0800 28 February 1998. Throughout the period there was a low pressure cell over the northwest of the con- tinent. A high pressure cell to the south of Australia continued east and was to the west of Victoria at the end of the period.

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